The present invention relates to noise reduction, and more particularly to a noise reduction circuit for reducing noise and smear which tend to develop in the luminance output of video tape recording and reproducing apparatus.
In a recording mode of color television video tape recorders, the luminance component of a video signal is passed through a noise reduction circuit of a recording section of the recorder to a preemphasis circuit to accentuate its higher frequency components against high-frequency noise which might be introduced during recording and is frequency-modulated before being fed to video heads, while the chrominance component is converted in frequency so that it occupies a lower frequency part of the video spectrum. In playback, the luminance signal is frequency-demodulated, deemphasized and fed to a noise reduction circuit of the playback section of the recorder before being combined with the chrominance component which has been reconverted to the original frequency.
As shown in FIG. 16, the noise reduction circuit of the recording section comprises a low-pass filter 110 to which the luminance signal R.sub.1, FIG. 17, is applied. The output R.sub.2 of low-pass filter 110 and the luminance signal R.sub.1 are combined in a subtractor 111 to supply a subtractor output to a multiplier 112 which multiplies the input signal by a prescribed factor, producing a signal R.sub.3. A limiter 113 limits the amplitude of the multiplier output R.sub.3 to produce an amplitude-limited signal R.sub.4 having an amplitude not exceeding a prescribed value L.sub.1. The limiter output R.sub.4 and the luminance signal R.sub.1 are summed in an adder 114 to produce an output R.sub.5, which is applied to the preemphasis circuit.
The noise reduction circuit of the playback section, which is shown in FIG. 18, comprises a noise rejection filter 120 which removes high-frequency noise introduced to a deemphasized luminance signal P.sub.1 to produce a noise-eliminated signal P.sub.2, FIG. 19. The noise-eliminated signal P.sub.2 is applied to a negative input of a first subtractor 121 which receives signal P.sub.1 at the positive input to provide subtraction of signal P.sub.2 from signal P.sub.1 to supply a first subtractor output P.sub.3 to a demultiplier 122 which multiplies the amplitude of the input signal by a factor which is determined in relation to the multiplying factor of the noise reduction circuit of the recording section. The output of demultiplier 122, indicated at P.sub.3, is applied to a limiter 123. Low-pass filter 120 and subtractor 121 constitute a high-pass filter which extracts the noise component contained in the reproduced luminance signal. Limiter 123 limits the amplitude of the extracted noise component P.sub.4 to a maximum value L.sub.1 to produce a limiter output P.sub.4 to eliminate high-amplitude peaks which would otherwise cause "smear" in the video screen. The amplitude-limited noise component P.sub.4 is applied to the negative input of a second subtractor 116 which subtracts it from the noise-contaminated signal Phd 1 to generate a noise-reduction output P.sub.5, which is applied to the mixer.
The cutoff frequency of low-pass filter 120 is set at a value normally in the range between 300 kHz and 1 MHz and the maximum amplitude of limiter 115 is set at a value in the range between 2% and 5% of the highest value of the luminance signal. As a result, the extracted noise component P.sub.4 has a duration as large as several hundreds nanoseconds and during this period the noise component cannot be removed. Furthermore, there appears a smear in the video screen to an extent determined by the maximum level of the limiter.
While the smear can be reduced by setting the cutoff frequency of low-pass filter 120 to a higher value and the noise can be reduced by setting the maximum level of the limiter as high as possible, the low-pass filter having a higher cutoff frequency would increase the noise, whereas the limiter with a higher maximum threshold would increase the smear. The usual practice is therefore to seek a compromise between these conflicting factors.